Proterozoic (about 2.5 billion years ago to about 570 million years ago)
Proterozoic era
Two stages in the late Precambrian. The stratum formed in this period is called Proterozoic, which is located above Archean and below Paleozoic. Proterozoic, formerly known as Proterozoic, was named by S.F. emmons in 1887. Proterozoic belongs to Greek etymology, which means early primitive life. Proterozoic is generally divided into three generations: Proterozoic, Mesoproterozoic and Neoproterozoic, and the boundaries are 65.438+0.8 billion years ago and 65.438+0 billion years ago respectively.
Compared with Proterozoic Archean, the metamorphic degree of rocks is shallow, and there are some metamorphic sedimentary rocks. Mainly slate, marble, dolomite, limestone, shale, sandstone and phyllite. The Proterozoic strata in China are complex, and the development degree varies greatly from place to place. The meso-Neoproterozoic strata in North China platform belong to stable caprock deposits, and the rest are active deposits. Red clastic sediments in Neoproterozoic strata are widely distributed in southern China.
In Proterozoic, algae and bacteria began to flourish, which was an important stage of evolution from prokaryotes to eukaryotes, unicellular protozoa to multicellular metazoa. Stromatolites first appeared in Archean, and the first development climax appeared in Proterozoic. In recent years, a kind of Chaetoceros fossil belonging to16 ~1700 million years ago has been found in Chuanlinggou Formation in northern China, which is the oldest eukaryote found so far. In late Proterozoic, invertebrates were occasionally found.
Proterozoic is an important metallogenic period, and the main minerals are iron, gold, uranium, manganese, copper, boron, phosphorus, magnesite and so on. Proterozoic isotope age is between 250-6 or (570) million years, and * * * has experienced a long time of 65.438+0.9 billion years. Proterozoic is divided into three generations. 25-/kloc-0.8 billion years is Proterozoic,/kloc-0.8-/kloc-0.0 billion years is Mesoproterozoic, and/kloc-0.0-6 years or (5.7 million) years is Neoproterozoic. Among them, the second half of Neoproterozoic, that is, 86 million years or 570 million years, is called Sinian. Proterozoic geological history has the following characteristics.
With the increasing abundance of algae, they constantly absorb CO2 in the atmosphere and release O2 through photosynthesis, which makes the atmosphere and water develop from anoxic state to more oxygen-containing state. Since Mesoproterozoic, iron-bearing purple sandstone (such as Changzhougou Formation and Dahongyu Formation, etc. ) and hematite beds (such as Long Xuan Iron Mine in Chuanlinggou Formation) have been formed, indicating that the atmosphere at that time already contained quite a lot of free oxygen. The increase of oxygen in the atmosphere and water not only affects the way and process of weathering and deposition of rocks, but also prepares material conditions for the development and evolution of organisms.
Fungi and cyanobacteria have appeared in Archean and further developed in Proterozoic. Blue-green algae are widely distributed in rock formations and form complexes through biological action and sedimentation. This complex is usually preserved in limestone and dolomite. Seen from the cross section, it is concentric ellipse. From the longitudinal section, it is an upward convex arc or cone-shaped stack, just like a pile of bowls locked together, called stromatolite.
The basic structural unit of stromatolite is called basal layer, which is generally arc-shaped or conical and convex upward. The basic layer is aggregate, the most common shapes are columnar, conical, rod-hammer, and some are wall-like (Figure 1 1-5). The aggregate has various bifurcation phenomena (Figure 1 1-6). Aggregates form large groups, which are mostly lenticular and layered reefs in the strata. Stromatolites are mainly distributed in the intertidal zone and supratidal zone along the coast, and some of them can be distributed in the depth of subtidal 100m. In recent years, stromatolites have been divided into many classes, groups and types according to their morphology, bifurcation form, body wall structure, ornamentation and internal structure, which has certain significance for stratigraphic division and correlation.
In addition, in recent years, tiny (often less than 10μm) micro-plants have been isolated from Proterozoic strata, mainly referring to some unicellular algae. In Late Proterozoic, the scale of micro-palaeophytes increased (50- 100 micron) and there were many kinds. Higher algae such as brown algae and red algae have also appeared since Mesoproterozoic. In recent years, the oldest eukaryotic fossil, named Chuaria, was found in the Mesoproterozoic Chuanlinggou Formation in northern China, dating back to16 ~1700 million years. Eukaryotic fossils were also found in the Mesoproterozoic Wumishan Formation in 1978, named Tawuia, which was12-1400 million years ago. The taxonomic status of these unicellular algae is unknown, and they are usually called suspected species. Archean from inanimate to living is a leap in the history of biological evolution, Proterozoic from prokaryotic to eukaryotic, from single cell to multicellular, marking a new stage in the history of earth development and life evolution.
After the late Archean tectonic movement-Fuping Movement, a small-scale stable nucleus called continental nucleus appeared in China and the world continent, which was the first stage of the development of continental crustal structure. The early Proterozoic tectonic movement was called Wutai Movement in China. The tectonic movement in the late Early Proterozoic is called Lvliang Movement in China. Through these movements, the continental core expanded further, forming a large-scale stable area called the original platform, on which sedimentary types similar to caprocks began to be deposited. Due to the repeated actions of sedimentation, eruption, intrusion, compression, folding, metamorphism and consolidation, some parts of the continental crust became more stable, and the original platform further expanded in the late Mesoproterozoic, and finally some large-scale stable ancient platforms appeared in the world. From continental core to primitive platform and ancient platform, it is the second stage of the development of continental crustal structure.
In terms of rock properties, the Paleoproterozoic strata, that is, the Lower Proterozoic, are often the same as the Upper Archean, and most of them belong to the greenstone series formed by active deposits and turbidite deposits. At the same time, like Upper Archean, they often contain huge iron deposits, which are similar in nature to Anshan-style iron deposits and mainly contain low-priced iron, reflecting the anoxic state of the atmosphere and water bodies at that time. The Lower Proterozoic (Pt 1) and Upper Archean (Ar2)*** together constitute the basement of the platform. In Mesoproterozoic and Neoproterozoic, the primitive platform has appeared, the shallow sea of the platform is stable, eukaryotes and algae are rich, the oxygen content in the atmosphere and water increases, and red beds, high-valent iron and carbonate deposits appear, forming platform cover. Therefore, Mesoproterozoic (Pt2), especially Sinian (Z) of Upper Proterozoic (Pt3), already belongs to the category of caprock deposition.
In Proterozoic, the North China Platform was formed in the north of China, the Yangtze Platform in the south and the Tarim Platform in the west. On the basis of initial consolidation, fault subsidence occurred in North China, forming clastic volcanic deposits represented by Hutuo Group and dolomite deposits containing stromatolites. The post-orogenic molasse deposit was formed in Wutai-Taihang Mountain area. These all belong to geosyncline active accumulation. Songshan Group was formed in Henan and Fengyang Group was formed in Anhui. They are all stable types, mainly composed of well-sorted clastic rocks and carbonate rocks, with few volcanic rocks. These deposits were flattened by folds and covered by Mesozoic and Neoproterozoic unconformities, which were widely distributed, namely the Lvliang Movement. After the Lvliang Movement, North China was further consolidated, forming the initial platform of North China.
In the southwest of China, including central Sichuan and western Hubei, the Archean Kongling Group metamorphic rock series above the basement is representative. The Lower Proterozoic is not widely distributed, mainly island arc volcanic sedimentary rock series, and there are similar deposits in western Sichuan. Later, the Lower Proterozoic and Upper Archean folds formed the Yangtze Platform.
The Lower Proterozoic in Tarim area of western China is dominated by shallow metamorphic volcanic sedimentary rock series, which belongs to active type and is called Xingditage Group. The in-situ platform was formed at the end of early Proterozoic.
At the end of Paleoproterozoic, relatively stable primitive platforms such as North China, Yangtze River and Tarim have appeared in China, but there are still great differences in continental crust stability.
1. As early as the end of Archean in North China, several stable continental cores were formed in North China and southern Liaoning, and there were active areas between the continental cores, which were filled with Wutai Group and Hutuo Group. After Lvliang movement, folding, metamorphism and consolidation, they connected the continental cores and formed a large-scale stable area-North China primitive platform. The terrain of this original station is undulating and quite complicated. Some areas began to sink again after long-term erosion, forming shallow seas; But some areas are higher than the sea surface, forming an ancient land, as shown in figure 1 1-7. The northern part of China belongs to the original station of North China except the northern part of Inner Mongolia and the northeast area. The original platform is roughly triangular, surrounded by highlands: Inner Mongolia ancient land in the north, Huaiyang ancient land in the south and Jiaodong ancient land in the east (later developed into Jiaoliao ancient land); There is a shallow sea on the land surface between ancient lands, and many mountains and land islands stand in the sea. For example, the ancient land in western Shandong and the ancient land in Shanxi and Shaanxi are larger ancient lands. In this shallow sea, the boundary of the upper platform similar to the caprock is deposited, so it is exposed in Liaoning, southern Jilin, Hebei, Shanxi (some areas), Daqingshan, Helan Mountain, central Shandong, western Henan and northern Anhui, but the sedimentary development varies from place to place. It can be roughly divided into three types: strong subsidence zone deposit, stable shallow sea deposit and continental deposit in uplift area.
(1) Liao Yan subsidence zone is located in the south of Inner Mongolia ancient land, which is a strong subsidence zone in North China. As shown in figure 1 1-8, the center of the depression is located in Xinglong, Hebei, Jixian, Tianjin and Pinggu, Beijing, with a sedimentary thickness of 10000m, complete strata development and clear stratification, which is the standard for the division and correlation of Mesoproterozoic in the north. Figure 1 1-9 is a comprehensive columnar section of the Middle-Upper Proterozoic in Pinggu area of Yanshan area.
On the profile of Figure 1 1-9, it can be divided into three series and 12 group according to sedimentary cycle, lithology and sedimentary discontinuity. The lower part of the Great Wall System is dominated by clastic rocks with volcanic eruption rocks and the upper part is carbonate rocks. The Jixian system is dominated by carbonate rocks with the largest thickness and wide distribution; Qingbaikou system is mainly composed of sand shale and limestone with small thickness and narrow distribution. The middle-upper Proterozoic represents a huge sedimentary cycle from bottom to top; In this cycle, it can be divided into three sub-first-order cycles, and there are obvious gaps between each cycle; Second-order cycles also contain smaller cycles. Accordingly, it shows that at the beginning of this era, represented by the profile, transgression began, huge thick clastic rocks were accumulated in the coastal shallow sea, and submarine volcanoes erupted. In the middle stage, the transgression expanded, and the surrounding areas overlapped (Figure 1 1- 10), forming a wide and thick carbonate stratum; In the later period, the crust rose, and the seawater gradually receded, mainly composed of clastic rocks. There is a second cycle or even a second cycle in the great cycle, which shows that the whole area is a wave-like development process.
The development process of the western Henan-Huainan subsidence zone located in the northern margin of Huaiyang ancient land is similar to that of Liao Yan subsidence zone.
(2) Other areas in North China, including Shandong, Henan and Anhui, are shallow seas with relatively stable land surface, with a small subsidence range and a deposition thickness of about1000 m.. The lower part is dominated by clastic phase and the upper part is dominated by carbonate phase.
(3) Shanxi-Shaanxi ancient land is surrounded by shallow sea, which is a long-term eroded uplift area, and its marginal part only forms thick coastal facies deposits when transgression overtakes it; Continental quartz sandstone is accumulated in inland lowlands, with good sorting property and developed staggered layers, and the thickness is only about 100m. ..